Researchers tested specially designed short proteins (peptides) against regular and drug‑resistant TB bacteria. The D‑form peptide called D5 stopped bacterial growth at low concentrations, and a similar peptide D1 worked almost as well but was less harmful to human cells. The natural human peptide LL‑37 didn’t do much against TB.

With the emergence of multi-drug resistant (MDR) and extensively drug resistant (XDR) Mycobacterium tuberculosis (Mtb), a new class of antimycobacterial agents with very different modes of action compared to classical antibiotics, are urgently needed. In this study, a series of 26-residue, amphipathic, α-helical antimicrobial peptides consisting of all D-amino acid residues and synthetic human L-LL37 (L-enantiomer) and D-LL37 (D-enantiomer) were investigated against M. tuberculosis susceptible strain (H37Rv) and a clinical multi-drug resistant strain (Vertulo). Minimal inhibitory concentrations (MICs) were determined through a peptide killing assay. D5, the most active analog against M. tuberculosis had a MIC value of 11.2 μM (35.2 μg/ml) against H37Rv strain and 15.6 μM (49 μg/ml) against the MDR strain. Peptide D1 had similar activity as D5 against the MDR strain (57 μg/mL), a 9-fold improvement in hemolytic activity and a 7.4-fold better therapeutic index compared to D5. Surprisingly, LL37 enantiomers showed little to no activity compared to the de-novo designed α-helical antimicrobial peptides.